NUTRITIVE METABOLISM 293 



organic matter, as saprophytes, or from living organisms, on which they 

 are parasitic. Certain plants, however, obtain a portion of their organic food 

 from the external world, while the rest is formed from carbonic acid and 

 water ; and many plants when adult obtain all their nutriment in this manner, 

 whereas when young the whole of their organic food was absorbed from 

 without (Sect. 64). There can be no doubt that we may ultimately succeed 

 in completely and satisfactorily replacing the sugar which the green plant 

 itself produces by introducing appropriate artificial food (Sect. 55). 



However important the photosynthetic production of organic material 

 may be in the nutrition of the green plant and in the balance of nature, 

 it is simply a special mode of obtaining organic nutriment, and the non- 

 chlorophyllous plant uses the sugar which it obtains from without in just 

 the same manner as the plant which can manufacture sugar for itself. 

 Moreover, the power of producing sugar is limited to the chloroplastids, 

 so that the plasma of a cell containing chlorophyll, and all the non- 

 chlorophyllous cells of the shoot and of the root, live upon the sugar 

 supplied to them in the same way as saprophytic or parasitic fungi, which 

 absorb the substances constructed from the sugar produced by green 

 plants, 



An animal is not always feeding, nor is a plant occupied continuously 

 in obtaining food, although, as in every organism, life involves unceasing 

 metabolism and a continuous liberation of energy, dependent for the most 

 part upon the processes of respiration which persist during day and night 

 in both animals and plants. In green plants, however, in the daytime 

 respiration is largely masked by the assimilation of carbon dioxide, which 

 is much more active under normal conditions than is the former. The total 

 amount of carbon assimilated must naturally be greater than that consumed 

 in respiration, in order that a green plant which obtains all its organic food 

 by its own assimilatory activity may be able to grow and increase in dry 

 weight. Not merely the organic food of all plants is derived from this 

 source, but the flesh and blood of animals is of similar origin, for carbon 

 dioxide and water, which are the ultimate products of decomposition, are 

 synthesized again by green plants by means of radiant energy from 

 the sun. 



The photosynthetic assimilation in the chloroplastid only provides the 

 organic food, which in green and non-green plants, and in animals also, has 

 the same function to perform. Whatever its source may be it provides 

 plastic and constructive material, and at the same time a supply of 

 potential energy. In all essential features, therefore, the metabolism of 

 plants resembles that of animals, for in both cases it is based upon chemical 

 changes involving a liberation or redistribution of energy ; and plants, like 

 animals, must sacrifice a large portion, and often almost the whole, of their 

 nutriment in order to provide a sufficient supply of kinetic energy. In 



